Women with a high percentage of fibroglandular tissue in their breast, as opposed to fatty tissue, have an increased breast cancer risk (up to 5-fold higher) in the subsequent 10 or more years. This attribute, known as mammographic density (MD), varies between women and can change within the same woman over time. Having both genetic and environmental determinants, between-country differences in MD may account for the over 6-fold international variations in breast cancer incidence rates. Only one study has investigated this to date, i.e. a US/Hawaii/Japanese comparison (Maskarinec et al. 2007). We aim to initiate a more widespread International Pooling Project of Mammographic Density to: (i) pool and obtain standardised comparable data on MD from countries spanning the breast cancer incidence range; (ii) describe international variations in overall and age-specific MD distributions and assess whether they are explained by individual-level risk factors for MD; (iii) quantify the extent to which international variations in MD correlated with corresponding breast cancer incidence rates and Pike's proposed model of breast tissue ageing; (iv) assess a range of MD metrics (absolute/relative/age-specific/cumulative); (v) continue and expand this pooled resource into the future. The International Pooling Project of Mammographic Density would be the first such initiative for MD, which could later be expanded to include new methods of MD measurement, imaging modalities and a broader range of determinants. Being led by the International Agency for Research on Cancer, we emphasize an international perspective for this marker, especially urgent as breast cancer is now the most common cancer in women in almost every country worldwide. If MD does underpin international variations in breast cancer incidence, the monitoring of MD-distributions would become an important early indicator of changes in breast cancer risk. PUBLIC HEALTH RELEVANCE: One of the strongest quantitative markers of breast cancer risk measurable on a mammogram in all women is mammographic density. The International Pooling Project on Mammographic Density will create the first standardized pooled data/imaging resource that includes women from all continents. It will be used to describe and account for international variations in mammographic density and to what extent they account for international variations in breast cancer incidence rates.